[clang] [Clang] Fix handling of overloads differing only by constraints and ref-qualifiers (PR #192018)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/192018

We should only error about inconsistent qualifiers if the functions are actually overloads.

Fixes #120812

>From bb3201d099130ab8eabc62d9de07fccaaedef10f Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Tue, 14 Apr 2026 09:57:57 +0200
Subject: [PATCH] [Clang] Fix handling of overloads differing only by
 constraints and ref-qualifiers.

We ashould only error about inconsistent qualifiers
if the functions are actually overloads.

Fixes #120812
---
 clang/docs/ReleaseNotes.rst     |  1 +
 clang/lib/Sema/SemaOverload.cpp | 18 ++++++++++--------
 clang/test/CXX/drs/cwg24xx.cpp  |  9 +++++++++
 3 files changed, 20 insertions(+), 8 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3e2d287d1eb1f..9e4699d642332 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -434,6 +434,7 @@ Bug Fixes to C++ Support
 - We no longer caches invalid variable specializations. (#GH132592)
 - Fixed an incorrect template argument deduction when matching packs of template
   template parameters when one of its parameters is also a pack. (#GH181166)
+- Clang no longer errors on overloads with different ref-qualifiers and constraints. (#GH120812)
 - Fixed a crash when a default argument is passed to an explicit object parameter. (#GH176639)
 - Fixed an alias template CTAD crash.
 - Fixed a crash when diagnosing an invalid static member function with an explicit object parameter (#GH177741)
diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 11e771bc240f1..5892a813c0f00 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -1521,6 +1521,8 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
     return false;
   };
 
+  bool ShouldDiagnoseInconsistentRefQualifiers = false;
+
   if (OldMethod && OldMethod->isExplicitObjectMemberFunction())
     OldParamsOffset++;
   if (NewMethod && NewMethod->isExplicitObjectMemberFunction())
@@ -1557,8 +1559,7 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
     }(OldMethod, NewMethod);
 
     if (!HaveCorrespondingObjectParameters) {
-      if (DiagnoseInconsistentRefQualifiers())
-        return true;
+      ShouldDiagnoseInconsistentRefQualifiers = true;
       // CWG2554
       // and, if at least one is an explicit object member function, ignoring
       // object parameters
@@ -1568,6 +1569,10 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
     }
   }
 
+  if (NewMethod && OldMethod && OldMethod->isImplicitObjectMemberFunction() &&
+      NewMethod->isImplicitObjectMemberFunction())
+    ShouldDiagnoseInconsistentRefQualifiers = true;
+
   if (!UseOverrideRules &&
       New->getTemplateSpecializationKind() != TSK_ExplicitSpecialization) {
     AssociatedConstraint NewRC = New->getTrailingRequiresClause(),
@@ -1582,12 +1587,6 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
       return true;
   }
 
-  if (NewMethod && OldMethod && OldMethod->isImplicitObjectMemberFunction() &&
-      NewMethod->isImplicitObjectMemberFunction()) {
-    if (DiagnoseInconsistentRefQualifiers())
-      return true;
-  }
-
   // Though pass_object_size is placed on parameters and takes an argument, we
   // consider it to be a function-level modifier for the sake of function
   // identity. Either the function has one or more parameters with
@@ -1612,6 +1611,9 @@ static bool IsOverloadOrOverrideImpl(Sema &SemaRef, FunctionDecl *New,
       return true;
   }
 
+  if(ShouldDiagnoseInconsistentRefQualifiers && DiagnoseInconsistentRefQualifiers())
+      return true;
+
   // At this point, it is known that the two functions have the same signature.
   if (SemaRef.getLangOpts().CUDA && ConsiderCudaAttrs) {
     // Don't allow overloading of destructors.  (In theory we could, but it
diff --git a/clang/test/CXX/drs/cwg24xx.cpp b/clang/test/CXX/drs/cwg24xx.cpp
index b27c6b823940c..0cbf2cdfdb966 100644
--- a/clang/test/CXX/drs/cwg24xx.cpp
+++ b/clang/test/CXX/drs/cwg24xx.cpp
@@ -212,5 +212,14 @@ struct T : S {
     virtual void k() &;
     virtual void l() &&;
 };
+#if __cplusplus >= 202002L
+
+// We should not error on inconsistent ref-qualifiers if the functions
+// have different constraints.
+template <unsigned R> struct type {
+    void func() const requires(R == 0);
+    void func() & requires(R == 1);
+};
+
 #endif
 }